Hydrophilic member and method for manufacturing same

ABSTRACT

In a hydrophilic member including a structure in which a photocatalytic TiO 2  layer and a porous SiO 2  layer are stacked on a surface of a base material, easy forming of the porous SiO 2  layer so as to be thin and have a uniform film thickness distribution that enables the porous SiO 2  layer to cover an entire surface of the photocatalytic TiO 2  layer, and enhancement in durability of the porous SiO 2  layer are enabled. A photocatalytic TiO 2  layer is formed so as to have a density of 3.33 to 3.75 g/cm 3  (preferably 3.47 to 3.72 g/cm 3 , more preferably 3.54 to 3.68 g/cm 3 ) on a surface of a base material. As an outermost surface layer, a porous SiO 2  layer is formed on the photocatalytic TiO 2  layer in such a manner that the porous SiO 2  layer has a film thickness of no less than 10 nm and no more than 50 nm.

TECHNICAL FIELD

The present invention relates to a hydrophilic member including astructure in which a TiO₂ (photocatalytic TiO₂) layer that providesphotocatalysis and a porous SiO₂ layer are stacked on a surface of abase material, and a method for manufacturing the same. In particular,the present invention enables easy forming of the porous SiO₂ layer thatis thin and has a uniform film thickness distribution that enables theporous SiO₂ layer to cover an entire surface of the photocatalytic TiO₂layer, and enhancement in durability of the porous SiO₂ layer.

BACKGROUND ART

Examples of a hydrophilic member including a structure in which aphotocatalytic TiO₂ layer and a porous SiO₂ layer are stacked on asurface of a base material are described in Patent Literatures 1 and 2.The hydrophilic member described in each of Patent Literatures 1 and 2is one that ensures hydrophilicity by means of the porous SiO₂ layer atan outermost surface, decomposes organic matter and the like adhered tothe porous SiO₂ layer by means of photocatalysis provided by thephotocatalytic TiO₂ layer below the porous SiO₂ layer, and therebyenables hydrophilicity of the porous SiO₂ layer to be maintained for along period of time.

CITATION LIST Patent Literature Patent Literature 1: Japanese PatentLaid-Open No. 10-36144 Patent Literature 2: Japanese Patent Laid-OpenNo. 2000-53449 SUMMARY OF INVENTION Technical Problem

In a hydrophilic member including the aforementioned structure, in orderto ensure uniform (that is, no irregularity through all regions of ahydrophilic surface of the hydrophilic member) and preferablehydrophilicity in every part of the hydrophilic surface, it is necessaryto form a porous SiO₂ layer that is thin and has a uniform filmthickness distribution that enables the porous SiO₂ layer to cover anentire surface of a photocatalytic TiO₂ layer when the film thickness ofthe porous SiO₂ layer is no more than 50 nm (preferably no more than 20nm). However, it is not easy to form a porous SiO₂ layer that is thinand has a uniform film thickness distribution on a photocatalytic TiO₂layer. In other words, in order to form a porous SiO₂ layer on aphotocatalytic TiO₂ layer by means of, for example, vapor deposition, itis necessary to vapor-deposit SiO₂ with an increased gas pressure(partial pressure of oxygen gas) in a vapor deposition atmospherecompared to that for cases where a non-porous vapor-deposited SiO₂ layeris formed. However, vapor deposition with an increased gas pressure in avapor deposition atmosphere causes instability in flight of SiO₂ vapordeposition molecules. Thus, irregularity in film thickness distributionoccurs in some regions of the hydrophilic surface, resulting in thephotocatalytic TiO₂ layer being partially exposed. Therefore,conventionally, in order to form an SiO₂ layer having a uniform filmthickness distribution, some ingenuity (e.g., in arrangement of acorrection plate and/or limiting the number of members to be subjectedto the film forming process at a time) in the film forming process isrequired.

The present invention is intended to solve the aforementioned problems.In other words, the present invention is intended to provide ahydrophilic member and a method for manufacturing the same that enableeasy forming of a porous SiO₂ layer that is thin and has a uniform filmthickness distribution that enables the porous SiO₂ layer to cover anentire surface of a photocatalytic TiO₂ layer, thereby preventing thephotocatalytic TiO₂ layer from being partially exposed, and enhancementin durability of the porous SiO₂ layer.

Solution to Problem

FIG. 2 indicates results of a test for measuring hydrophilicity recoverytime of a hydrophilic member. The hydrophilic member used in this testis one formed by forming a photocatalytic TiO₂ layer on a surface of aflat and smooth base material and forming an SiO₂ layer having a filmthickness of no more than 50 nm obtained by vapor-depositing SiO₂ vapordeposition molecules on the photocatalytic TiO₂ layer with a low gaspressure that enables stable flight of the SiO₂ vapor depositionmolecules. Samples of the hydrophilic member with respectivephotocatalytic TiO₂ layers having different densities were prepared, andfor each sample, time from a state in which hydrophilicity had been lostdue to adherence of organic matter to a surface thereof to recovery ofhydrophilicity due to ultraviolet irradiation (hydrophilicity recoverytime) was measured. In this test, a surface of the SiO₂ layer of eachsample was contaminated by an oil to lose hydrophilicity of the surface,and then, the surface is irradiated with an ultraviolet ray having anintensity of 1 mW/cm² using a black light. Recovery of hydrophilicitywas determined when a water droplet contact angle was decreased to becomparable to an initial value before the contamination (no more thanfive degrees). Also, whether or not SiO₂ vapor deposition moleculesstably fly during preparation of the samples can be determined based on,for example, whether or not a current (emission current) value of anelectron beam or a vapor deposition speed during vapor deposition isstable. In this case, the vapor deposition speed can be measured as, forexample, a derivative value of an oscillation frequency of aquartz-crystal film thickness meter. Also, the density of thephotocatalytic TiO₂ layer of each sample can be adjusted by film formingconditions (e.g., a temperature of the base material, the film formingspeed and/or the gas pressure), and the density can be measured by meansof, for example, grazing incidence X-ray diffractometry. As can be seenfrom FIG. 2, as the density of the photocatalytic TiO₂ layer is lower,the hydrophilicity recovery time is shorter, and where the densityexceeds 3.68 g/cm³, the hydrophilicity recovery time drasticallyincreases, and where the density exceeds 3.75 g/cm³, the hydrophilicityrecovery time becomes too long, the photocatalytic TiO₂ layer becomesimpracticable. The hydrophilicity recovery time being short means thatphotocatalysis provided by the photocatalytic TiO₂ layer easily reachesthe surface of the SiO₂ layer because the SiO₂ layer is porous. Thehydrophilicity recovery time being long means that it is hard forphotocatalysis provided by the photocatalytic TiO₂ layer to reach thesurface of the SiO₂ layer because the SiO₂ layer is nonporous. Theresults of this test indicate that forming a photocatalytic TiO₂ layerso as to have a density of no more than 3.75 g/cm³ (preferably no morethan 3.72 g/cm³, more preferably no more than 3.68 g/cm³) that is lowerthan 3.90 g/cm³, which is a typical density of anatase crystalstructures, enables forming of a porous SiO₂ layer even if SiO₂ vapordeposition molecules is vapor-deposited on the photocatalytic TiO₂ layerwith a low gas pressure that enables stable flight of the SiO₂ vapordeposition molecules. Since the vapor-deposition can be performed with alow gas pressure, a porous SiO₂ layer that is thin and has a uniformfilm thickness distribution can easily be formed with no specialingenuity in the film forming process. The test conducted by the presentinventors indicates that where a photocatalytic TiO₂ layer having adensity of no more than 3.75 g/cm³ is formed and SiO₂ vapor depositionmolecules are vapor-deposited on the photocatalytic TiO₂ layer with alow gas pressure that enables stably flight of the SiO₂ vapor depositionmolecules, a porous SiO₂ layer is formed. Also, if the porous SiO₂ layerhas a thickness of no less than 10 nm, an entire surface of thephotocatalytic TiO₂ layer can be covered by the porous SiO₂ layer (thatis, partial exposure of the photocatalytic TiO₂ layer can be prevented).

FIG. 3 indicates results of a test in which for each of samples that aresimilar to those used in the test in FIG. 2 (samples of a hydrophilicmember formed by forming a photocatalytic TiO₂ layer on a surface of aflat and smooth base material and forming an SiO₂ layer having a filmthickness of no more than 50 nm obtained by vapor-depositing SiO₂ vapordeposition molecules on the photocatalytic TiO₂ layer with a low gaspressure that enables stable flight of the SiO₂ vapor depositionmolecules, the samples including respective photocatalytic TiO₂ layershaving different densities), a scratching load for the SiO₂ layer wasmeasured. This test was conducted according to a procedure that issimilar to that of a pencil hardness test, using an iron rod instead ofa pencil, by measuring a load with each of weights having differentweights employed. FIG. 3 indicates that as the density of thephotocatalytic TiO₂ layer is lower, the SiO₂ layer formed on thephotocatalytic TiO₂ layer is more brittle, and as the density of thephotocatalytic TiO₂ layer is higher, the SiO₂ layer is harder.

FIG. 4 indicates results of a test in which for each of samples that aresimilar to those used in each of the tests in FIGS. 2 and 3, anacid-resistance of the SiO₂ layer was measured. This test was conductedby dropping H₂SO₄ having a concentration that is a normality of 0.1N ona surface of the SiO₂ layer and observing the state of the surface afterbeing left for 24 hours. In this test, where the density of thephotocatalytic TiO₂ layer is less than 3.33 g/cm³, a color of a part onwhich H₂SO₄ was dropped was more faded compared to a color of a partsurrounding that part. This is because the base material was exposed asa result of the SiO₂ layer and the photocatalytic TiO₂ layer beingstripped off at that part, resulting in no interference color generatedby the SiO₂ layer and the photocatalytic TiO₂ layer. On the other hand,where the density of the photocatalytic TiO₂ layer is no less than 3.33g/cm³, at the part on which H₂SO₄ was dropped, no fading occurred andthe SiO₂ layer and the photocatalytic TiO₂ layer were not stripped off.Therefore, the test results in FIG. 4 indicate that where the density ofthe photocatalytic TiO₂ layer is less than 3.33 g/cm³, theacid-resistance is low and if the density of the photocatalytic TiO₂layer is no less than 3.33 g/cm³, the acid-resistance is high.

The test results in FIGS. 3 and 4 indicate that forming a photocatalyticTiO₂ layer having a density of no less than 3.33 g/cm³ (preferably noless than 3.47 g/cm³, more preferably no less than 3.54 g/cm³) enablesprovision of a practical durability (scratch resistance and acidresistance).

Accordingly, the results of the tests in FIGS. 2 to 4 indicate thatforming a photocatalytic TiO₂ layer having a density of 3.33 to 3.75g/cm³ (preferably 3.47 to no more than 3.72 g/cm³, more preferably 3.54to 3.68 g/cm³) enables easy forming of a porous SiO₂ layer that is thinand has a uniform film thickness distribution that enables the porousSiO₂ layer to cover an entire surface of the photocatalytic TiO₂ layerand enhancement in durability of the porous SiO₂ layer.

Therefore, in the present invention, a photocatalytic TiO₂ layer havinga density of 3.33 to 3.75 g/cm³ (preferably 3.47 to no more than 3.72g/cm³, more preferably 3.54 to 3.68 g/cm³) is formed on a surface of abase material and, as an outermost surface layer, a porous SiO₂ layerhaving a film thickness of no less than 10 nm and no more than 50 nm(preferably no less than 15 nm and no more than 20 nm) is formed on theTiO₂ layer in such a manner that the porous SiO₂ layer covers an entiresurface of the TiO₂ layer. Consequently, a thin porous SiO₂ layer can beformed on a photocatalytic TiO₂ layer and can also be formed so as tohave a uniform film thickness distribution that enables the porous SiO₂layer to cover an entire surface of the photocatalytic TiO₂ layer,enabling provision of favorable and uniform photocatalysis by thephotocatalytic TiO₂ layer. Also, the durability of the porous SiO₂ layercan be enhanced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional diagram illustrating an embodimentof a hydrophilic member of the present invention.

FIG. 2 is a diagram indicating results of a test in which for each ofsamples of a hydrophilic member (samples of a hydrophilic member formedby forming a photocatalytic TiO₂ layer on a surface of a flat and smoothbase material and forming an SiO₂ layer having a film thickness of nomore than 50 nm obtained by vapor-depositing SiO₂ vapor depositionmolecules on the photocatalytic TiO₂ layer with a low gas pressure thatenables stable flight of the SiO₂ vapor deposition molecules, thesamples including respective photocatalytic TiO₂ layers having differentdensities), time from a state in which hydrophilicity had been lost dueto adherence of organic matter to a surface thereof to recovery ofhydrophilicity due to ultraviolet irradiation was measured.

FIG. 3 is a diagram indicating results of a test in which for each ofsamples that are similar to those used in the test in FIG. 2, ascratching load for the SiO₂ layer was measured.

FIG. 4 is a chart indicating results of a test in which for each ofsamples that are similar to those used in each of the tests in FIGS. 2and 3, an acid-resistance of the SiO₂ layer was measured.

FIG. 5 is a schematic diagram illustrating an example of a vacuum vapordeposition apparatus 18 for manufacturing the hydrophilic member 10 inFIG. 1.

DESCRIPTION OF EMBODIMENT

FIG. 1 is a schematic cross-sectional diagram of an embodiment of ahydrophilic member of the present invention. A hydrophilic member 10 isconfigured by forming a photocatalytic TiO₂ layer 14 on a flat andsmooth surface of a base material 12 and, as an outermost surface layer,forming a porous SiO₂ layer 16 on the photocatalytic TiO₂ layer 14. Theporous SiO₂ layer 16 is formed so as to have a uniform film thicknessdistribution that enables the porous SiO₂ layer 16 to cover an entiresurface of the photocatalytic TiO₂ layer. A density of thephotocatalytic TiO₂ layer 14 is 3.33 to 3.75 g/cm³ (preferably 3.47 tono more than 3.72 g/cm³, more preferably 3.54 to 3.68 g/cm³). A filmthickness of the photocatalytic TiO₂ layer 14 is 50 to 500 nm. A filmthickness of the porous SiO₂ layer 16 is no less than 10 nm and no morethan 50 nm (preferably no less than 15 nm and no more than 25 nm).

The hydrophilic member 10 enables provision of, for example, anautomobile window, or building window glass by forming the base material12 using a transparent glass plate or a transparent resin plate. Also,the hydrophilic member 10 enables provision of, for example, a backsurface mirror-type outer mirror for a vehicle or a back surface mirrorsuch as a bathroom mirror by forming the base material 12 using atransparent glass plate or a transparent resin plate and forming areflective film on a back surface of the base material 12. Also, thehydrophilic member 10 enables provision of, for example, a front surfacemirror such as a front surface mirror-type outer mirror for anautomobile by forming the base material 12 using a glass plate or aresin plate and forming a reflective film between the base material 12and the photocatalytic TiO₂ layer 14. Also, the hydrophilic member 10enables provision of a fog-resistant optical element by forming the basematerial 12 using an optical element such as a lens. If the basematerial 12 is a glass plate, a block layer (barrier layer) of, e.g.,SiO₂ can additionally be arranged between the base material 12 and thephotocatalytic TiO₂ layer 14 in order to prevent diffusion of alkaliions in the base material 12 into the photocatalytic TiO₂ layer 14.

An example of a method for manufacturing the hydrophilic member 10 inFIG. 1 will be described. Here, the base material 12 is formed using aglass plate, and each of the photocatalytic TiO₂ layer 14 and the porousSiO₂ layer 16 is formed by means of vapor deposition.

FIG. 5 illustrates an example of a vacuum vapor deposition apparatus 18.A vacuum chamber 20 is evacuated by a diffusion pump 22 and a rotarypump 24. In an upper portion of the vacuum chamber 20, a substrateholder 26 is arranged, and a glass plate 12, which forms a base materialfor the hydrophilic member 10, is held by the substrate holder 26, witha film-forming surface directed downward. The substrate holder 26 isheated by a heater 28 and the glass plate 12 is kept at a desiredtemperature via the substrate holder 26. A crucible 30 is arranged at aposition below the glass plate 12, and a vapor deposition material(starting substance for vapor deposition) 32 is placed in the crucible30. Examples of the vapor deposition material 32 for forming a TiO₂layer 14 include, e.g., TiO₂, Ti₂O₃ and Ti. Examples of the vapordeposition material 32 for forming an SiO₂ layer 16 include, e.g., SiO₂and SiO.

The vapor deposition material 32 is evaporated as a result of beingirradiated with an electron beam 36 emitted from a hot cathode 34. As areactive gas, an oxygen gas 42 is introduced from an oxygen tank 40 intothe vacuum chamber 20. The evaporated vapor deposition material 32reacts with the oxygen gas 42 to produce TiO₂ or SiO₂. The produced TiO₂or SiO₂ is deposited on a surface of the glass plate 12, whereby a TiO₂layer 14 or an SiO₂ layer 16 is formed. A film thickness during the filmforming is monitored by a film thickness monitoring apparatus 44, andthe vapor deposition is stopped when a desired film thickness isreached.

Film properties of the vapor-deposited film vary depending on, e.g., thetemperature of the glass plate 12, the vapor deposition speed and thepartial pressure of the oxygen gas 42 in the vacuum chamber 20. Anexample of film forming conditions for forming a photocatalytic TiO₂layer having a density of 3.33 to 3.75 g/cm³ and forming a porous SiO₂layer 16 on the photocatalytic TiO₂ layer 14 having a uniform filmthickness distribution that enables the SiO₂ layer 16 to cover an entiresurface of the photocatalytic TiO₂ layer if the film thickness of theporous SiO₂ layer 16 is no less than 10 nm is indicated in the followingtable.

Photocatalytic Porous SiO₂ TiO₂ layer 14 layer 16 Temperature of 300degrees centigrade 300 degrees centigrade glass plate 12 Vapor 0.5nm/sec. 0.2 nm/sec. deposition speed Partial pressure 0.016 Pa 0.016 Paof oxygen gas 42

An example of a procedure for forming a photocatalytic TiO₂ layer 14 anda porous SiO₂ layer 16 using the vacuum vapor deposition apparatus 18 inFIG. 5 will be described below. A photocatalytic TiO₂ layer 14 isformed, for example, according to the following procedure.

(1) Hold a glass plate 12 in the substrate holder 26, place, forexample, Ti₂O₃ as a vapor deposition material 32 in the crucible 30, andclose the vacuum chamber 20.(2) Drive the rotary pump 24 and the diffusion pump 22 to evacuate thevacuum chamber 20.(3) Drive the heater 28 to heat the glass plate 12 to a predeterminedtemperature through the substrate holder 26.(4) Introduce an oxygen gas 42 from the oxygen tank 40 into the vacuumchamber 20.(5) Drive the hot cathode 34 to irradiate the Ti₂O₃, which is a vapordeposition material 32, with an electron beam 36 to evaporate the Ti₂O₃.(6) The evaporated Ti₂O₃ reacts with the oxygen gas 42 to produce TiO₂.The produced TiO₂ is deposited on the glass plate 12, whereby a TiO₂film is formed.(7) End the film forming when approximately 100 nm of TiO₂ is deposited.

Upon the end of the forming of the photocatalytic TiO₂ layer 14,subsequently, a porous SiO₂ layer 16 is formed. A porous SiO₂ layer 16is formed, for example, according to the following procedure.

(1) Place, for example, SiO₂ as a vapor deposition material 32 in thecrucible 30 and close the vacuum chamber 20.(2) Drive the rotary pump 24 and the diffusion pump 22 to evacuate thevacuum chamber 20.(3) Drive the heater 28 to heat the glass plate 12 to a desiredtemperature through the substrate holder 26.(4) Introduce an oxygen gas 42 from the oxygen tank 40 to the vacuumchamber 20.(5) Drive the hot cathode 34 to irradiate the SiO₂, which is a vapordeposition material 32, with an electron beam 36 to evaporate the SiO₂.(6) The evaporated SiO₂ is deposited on the photocatalytic TiO₂ layer 14on the glass plate 12, whereby a SiO₂ film is formed.(7) End the film forming when approximately 15 nm of SiO₂ is deposited.

Since an outermost surface of the hydrophilic member 10 produced by theabove process include the porous SiO₂ layer 16 alone, the hydrophilicmember 10 exerts excellent effects in surface hardness andhydrophilicity maintenance compared to cases where the outermost surfaceincludes a photocatalytic TiO₂ layer alone or a layer of a mixture ofphotocatalytic TiO₂ and SiO₂.

Although the above embodiment has been described in terms of a casewhere a photocatalytic TiO₂ layer and a porous SiO₂ layer are formed bymeans of vapor deposition, it can be considered that the effects of theinvention according to the present application can also be expectedwhere both or one of the layers is formed by means of another thin filmforming method (for example, sputtering).

REFERENCE SIGNS LIST

10 . . . hydrophilic member, 12 . . . base material, 14 . . .photocatalytic TiO₂ layer, 16 . . . porous SiO₂ layer

1. A hydrophilic member comprising a structure in which a TiO₂ layerthat provides photocatalysis is formed so as to have a density of 3.33to 3.75 g/cm³ on a surface of a base material, and a porous SiO₂ layeris formed as an outermost surface layer on the TiO₂ layer in such amanner that the porous SiO₂ layer has a thickness of no less than 10 nmand no more than 50 nm and covers an entire surface of the TiO₂ layer.2. The hydrophilic member according to claim 1, wherein the density ofthe TiO₂ layer is 3.47 to 3.72 g/cm³.
 3. The hydrophilic memberaccording to claim 2, wherein the density of the TiO₂ layer is 3.54 to3.68 g/cm³.
 4. The hydrophilic member according to claim 1, wherein thefilm thickness of the porous SiO₂ layer is no less than 15 nm and nomore than 20 nm.
 5. The hydrophilic member according to claim 2, whereinthe film thickness of the porous SiO₂ layer is no less than 15 nm and nomore than 20 nm.
 6. The hydrophilic member according to claim 3, whereinthe film thickness of the porous SiO₂ layer is no less than 15 nm and nomore than 20 nm.
 7. A hydrophilic member manufacturing method comprisingthe steps of: forming a TiO₂ layer that provides photocatalysis, so asto have a density of 3.33 to 3.75 g/cm³ on a surface of a base material;and forming a porous SiO₂ layer as an outermost surface layer on theTiO₂ layer in such a manner that the porous SiO₂ layer has a thicknessof no less than 10 nm and no more than 50 nm and covers an entiresurface of the TiO₂ layer.